High performance gas chromatographs traditionally have been limited to use in the laboratory because of power, size and weight constraints imposed on field instruments. Further, problems have been associated relative to the use of cryogenic liquids to achieve sample focusing, limited temperature programming rates, and accurate temperature control of the chromatograph column.
The use of directly heated gas chromatograph columns has been previously suggested for a nonoxidizable steel column as shown generally in French Patent No. 2,344,838. Further, direct heating has been suggested for a metal coated fused silica capillary column. In fact, Cates et al U.S. Pat. No. 4,726,822 describes a fast response thermochromatographic capillary column comprising a fused silica capillary with a high temperature chemically inert polymer coating thereon supporting a thin heater film formed by the deposition of either a high resistance metallic compound or a nichrome film sputtered onto the polymer clad capillary tubing. Such a column is directly heated by passing a current through the film. However, this patent does not solve the problem of accurately controlling the temperature, or providing an appropriate low thermal mass support for the resistively heated column.
Gas chromatograph systems have used stationary phase focusing to achieve sample bandwidth narrowing. With more volatile species cryogenic enhancement improves the focusing of the volatile species Cryogenic focusing on short column sections or on whole columns has been known utilizing liquid nitrogen. However, use of liquid nitrogen is not practical on portable units.
The present invention is intended to overcome one or more of the problems as set forth above.